The invention relates to a non-scaling heat treatable steel. The invention also relates to a method for producing a non-scaling component according to claim 5 and the production of a strip made of this steel.
Such components are produced from pre-products such as sheet metals, metal plates seamless or welded tubes and are mainly used in the automobile industry, but also in the agricultural machine construction for example for plowshare, in the construction industry, for example for wear plates or in wind energy systems for example as support structures.
As is known, heat treatment of a component is achieved by austenizing, quenching and subsequent tempering of the steel material, wherein depending on the field of use, components are also used solely in the hardened, i.e., non-tempered state.
The hotly contested market of the automobile industry forces automobile producers to constantly seek solutions for lowering the fleet consumption while maintaining a highest possible comfort and passenger protection. In this regard the weight saving of all vehicle components plays an important role in one hand, but on the other hand also a most advantageous behavior of the individual components at high static and dynamic stress during operation and in the event of a crash.
Suppliers of starting materials seek to account for this requirement by making available high and ultra-high strength steels thereby enabling reducing the wall thicknesses while at the same time improving component behavior during manufacture and during operation.
These steels therefore have to meet relatively high demands regarding strength, ductility, tenacity, energy absorption capacity and corrosion resistance as well as regarding their processability for example during cold forming and during welding.
In light of the aforementioned aspects the production of components from hot formable and press hardened steels gains increasing importance because these ideally satisfy the increased demands on the component properties at low material costs.
The production of press hardened components by means of quenching of pre: products made of press hardenable steels by hot forming in a forming tool is known from DE 601 19 826 T2. Here a steel plate which is heated beforehand to above austenizing temperature to 800-1200° C. and provided with a coating of zinc or zinc basis is formed in a tool, which in some cases may be cooled, to a component wherein the sheet metal or component is subjected during the forming to a quenching (press hardening) by fast heat withdrawal and thereby achieves the demanded strength properties. The metallic coating here acts as oxidation or scale protection.
The production of components by means of quenching of pre-products made of press hardenable steels by hot forming in a forming tool is also known from DE 699 33 751 T2. Here, a steel sheet is coated with a metallic coating made of an aluminum alloy, heated prior to a forming to above 700° C., wherein an intermetallic alloyed compound on the basis of iron, aluminum and silicone is generated on the surface and the sheet metal is then formed and cooled with a rate above the critical hardening speed. The metallic coating also in this case acts a oxidation or scaling protection.
The application of oxidation or scaling protection onto the pre: product to be formed prior to the heating to forming temperature is advantageous in the known press form hardening because the coating allows effectively avoiding or even preventing scaling of the basic material and tool wear.
Without such a protection the heated pre-products would scale when coming into contact with oxygen of the atmosphere and the tools would be exposed to strong wear. The industrially used heating furnaces are usually operated with an atmosphere that is not oxidizing for iron, however when the plate is transported from the furnace into the die a strong scaling occurs at the ambient atmosphere. Prior to further processing the components have to be descaled by costly blasting.
The metallic coating, which acts as oxidation or scaling protection, is usually applied to the hot or cold strip in the continuous process. In hot dip coatings this can for example be a hot dip galvanizing or hot dip aluminizing. It is also known to use a varnish based non-metallic coating instead of a metallic coating. It is also known to use an electrolytically deposited metallic layer made of zinc and nickel.
Known hot formable heat treatable steels for use in the automobile industry are for example the known manganese boron steel “22MnB5” and recently also air-hardenable steels according to a still unpublished patent application of the applicant.
The production of such components by press form hardening of pre-products form the known materials has however several disadvantages.
When a coating or a cover for avoiding scaling is desired during heating to forming temperature, the production costs significantly increase for such steels. In addition resources are used up and the environment is negatively affected by the increased energy consumption.
Because the forming above AC3 temperature is usually significantly above 800° C., extremely high demands are also placed on the temperature resistance of a protection against scaling. In case of a protection against scaling on zinc basis there is also the risk of liquid metal embrittlement.
A disadvantage is also the processing of press hardenable steels with a coating or a cover in and of itself because certain holding or furnace times have to be observed during heating to forming temperature, which limits the flexibility in the process sequence on the customer side. In addition the scrap rates increase, because for example a plate an no longer be used when due to malfunction the furnace time is increased.
But also in the case of pre-products that are brought into a hardened or tempered condition solely via a corresponding temperature profile without forming and are subsequently further processed to a component, the scaling of the work piece surface has to be laboriously prior to further processing, which also significantly increases production costs.
From DE 36 04 789 C1 heat treatable steels are known which have the problem that at Al contents of more than 0.015% the required austenizing temperatures with 950 to 1050° C. are very high and with this a strong scaling is associated. In order to ensure the hardenability also at lower temperatures zircon is added to the steel in amounts that are adjusted to the nitrogen content, in order to prevent aluminum nitride precipitations in the steel, which were recognized as negatively influencing sufficient hardenability. The heat treatable steels A-H with good heat treatable properties tested there in table 1 have the following alloy composition in weight %: C: 0.32-0.75, Si: 0.26-0.37, Mn: 0.40-1.50, P: 0.009-0.012, Si: 0.005-0.012, Al: 0.016-0.022, Cr: 0.02-1.52, Zr: 0.035-0.060, N: 0.0042-0.0065.